1. Field of the Invention
The present invention relates to an optical disk reproducing device, and, more specifically, relates to an optical disk reproducing device which permits to restore in a short time from an out of focus servo to a focus servo condition.
2. Conventional Art
Recently, with regard to CD-ROM devices, the data read out speed thereof is rapidly increasing, for example, from a double speed to 32 times speed or more than that speed, for this reason, it is required to read data in a speed range from double speed to 32 times speed or more that speed. Further, also with regard to optical disk memory devices, the memory density thereof is rapidly increasing, for example, to double, four times, six times, eight times and so on.
For these sorts of the optical disk reproducing devices, a focusing servo mechanism with a very high accuracy is required.
A focus servo device in a CD reproducing device receives reflection beams from a CD by a tetra-section photo detector (pick-up) in a pick-up unit and generates, through a focus position error generation circuit, RF detection signals representing detection signals of total optical beam amount received at the respective optical beam receiving areas (which are detection signals representing an optical beam amount received by the photo detector of the reflection beams from the optical disk, in other words, signals representing reflection level of the reflection beams from the optical disk, and hereinbelow are called as RF detection signals) and a position error detection signal FE (FE is an abbreviation of Focus Error) representing a detection signal determined by a mutual difference of the respective sums of detection signals of the opposing optical beam receiving areas.
Then, through level detection of the position error detection signal FE or the RF detection signals it is detected whether the operation enters into a just focus control range. When the position error detection signal FE or the RF detection signal reaches a just focus control level, in case of an analogue control, a focus servo loop is formed which includes a pick-up unit, an amplifier (or a gain adjusting use amplifier) in the focus position error generation circuit, a servo filter and a focus position adjusting mechanism (lens moving mechanism) in the pick-up unit, and the loop is automatically controlled so that the position error detection signal FE is rendered to be zero, thereby, a focusing of the laser beams outputted from the pick-up unit is performed so that the irradiation surface of the optical disk assumes a focusing position of the laser beams. Such focusing is performed, for example, by moving a lens disposed in an optical path for laser beams in the focus position adjusting mechanism into a focusing position.
Like operation is performed in case of a digital control, in that in the digital control processings corresponding to the functions of the respective circuits as has been explained above in connection with the analogue control are realized by executing predetermined corresponding programs through a MPU. Therein, a signal value which renders the position error detection signal FE to zero is calculated, the calculated error correction value is converted into an analogue value via a D/A converter and sends out the analogue signal, which is converted so as to focus the laser beams onto the irradiation surface of the optical disk, to the focus position adjusting mechanism as a driving signal.
FIG. 3 is a block diagram for explaining such sort of a conventional focus servo device.
In FIG. 3, numeral 1 is a focus servo device, numeral 2 is an optical disk therefor (illustration of which rotation mechanism is omitted), numeral 3 is a pick-up unit which includes a tetra-section photo detector (pick-up), a laser beam source, a lens disposed in an optical path of the laser beams and a focus position adjusting mechanism for moving the lens in back and forth direction, and through the movement of the lens a focusing position of the laser beams outputted from the laser beam source is moved in back and forth direction with respect to the optical disk 2.
Numeral 4 is a driver, when a drive signal is received, the driver 4 moves the lens in the focus position adjusting mechanism in back and forth direction.
Numeral 5 is an RF amplifier which includes inside thereof a focus position error generation circuit, receives a detection signal from the tetra-section photo detector (pick-up) in the pick-up unit 3 and outputs the RF detection signal and the position error detection signal FE.
Numeral 6 is a just focus detection circuit which receives clocks CLK from a clock circuit 11 and the position error detection signal FE with respect to a focus point, detects in synchronism with the clocks CLK the zero crossing point of the position error detection signal FE as well as detects whether the level of the position error detection signal FE is in a predetermined range (which corresponds to a focus position covered by the automatic focus control loop) with reference to the zero crossing point to detect a just focus position and sends out a detection signal Dj to a focus controller 7. The just focus detection circuit 6 continuously generates the detection signal Dj so long as the level of the position error detection signal FE at this moment is kept at the above predetermined range.
The focus controller 7 receives the clocks CLK from the clock generation circuit 11 and the just focus position detection signal Dj, generates in synchronism with the clocks CLK a control signal LOOPON representing a servo loop on signal and outputs the same to a change-over circuit 9 to change over to the side A of servo control (the condition as illustrated). Thereby, the focus servo device 1 enters into an automatic focus control loop (just focus control). Further, the focus controller 7 continuously generates the control signal LOOPON during when the just focus position detection signal Dj is received and when no just focus position detection signal Dj is received, the generation of the control signal LOOPON is stopped. When no control signal LOOPON is generated, the change-over circuit 9 is changed over to the side B of focus search. Further, at this instance, the focus controller 7 sends out a control signal SS to a search signal generation circuit 8 to operate the same.
A servo filter circuit 10 receives the position error detection signal FE from the RF amplifier 5, performs a servo filter processing (which is a processing performed through a phase compensation filter, usually a loop filter is used therefor) for correcting a position error with respect to focusing, sends out the output to the change-over circuit 9 and outputs the same via the side A of servo control to the driver 4. As a result, a servo loop for the automatic focus control is formed with the pick-up unit 3, and the laser beams are focused onto the irradiation face of the optical disk 2.
Numeral 8 is the search signal generation circuit which is constituted by an up and down counter 81 and a D/A converter 82 and is operated by the control signal SS from the focus controller 7, wherein the up and down counter 81 counts the clocks CLK from the clock generation circuit 11 and the D/A converter 82 converts the count value into an analogue value to generate a ramp voltage. The generated ramp voltage is applied to the change-over circuit 9 and then applied to the driver 4 via the side B of focus search. Thereby, the focus position of the laser beams irradiated from the pick-up unit 3 onto the face of the optical disk 2 is continuously moved in backward or forward.
As an overall operation of the device, at first the change over circuit 9 is set to the side B of focus search by the focus controller 7, and the focus controller 7 activates with the control signal SS the search signal generation circuit 8 which generates the ramp voltage and drives the driver 4. Thereby, the driver 4 drives the focus position adjusting mechanism in the pick-up unit 3 depending on the ramp voltage to move the lens. As a result, the focusing position of the laser beams irradiated from the laser beam source onto the optical disk 2 rises from the lowest point. Thereafter, at the moment when the just focus position is detected by the just focus detection circuit 6 receiving the position error detection signal FE, the focus controller 7 generates the control signal LOOPON to change-over the change-over circuit 9 to the side A of servo control. As a result, the automatic focusing servo loop is formed by the pick-up unit 3, the RF amplifier 5, the servo filter circuit 10 and the driver 4, thereby, the laser beams outputted from the pick-up unit 3 are focused onto the irradiation face of the optical disk 2.
Now, during the conventional focus control as has been explained above, if the device is subjected to such as external vibration, the optical beam receiving position of the tetra-section photo detection is shifted and the level of the position error detection signal FE outputted from the pick-up unit 3 varies, thereby, the automatic focusing servo loop can not be maintained. When the maintenance of the automatic focusing servo loop is failed, another focusing control is again required.
FIG. 4 is a diagram for explaining this focusing control.
The ordinate is a movement range of a focus of laser beams outputted from the pick-up unit 3 for the optical disk 2 and the abscissa is time t.
Period (1) is a period during which a first focusing search is performed. Thereafter, when the detection signal Dj is generated and the focus of the laser beams reaches the position J (focusing position) of the just focus detection point, the automatic focusing servo loop is formed to maintain the position (focusing position), which represents period (2). During this period the detection signal Dj is continuously generated from the just focus detection circuit 6.
Herein, if the device is subjected to such as an external vibration, the optical beam receiving position of the tetra-section photo detector is shifted to move out from the automatic focusing servo loop. If the control goes out from the automatic focusing servo loop, the detection signal Dj is stopped and the output LOOPON of the focus controller 7 is stopped. Accordingly, another focus search is performed of which period corresponds to period (3). Then, an automatic focusing servo loop is again formed and a focus of the laser beams is maintained on the focusing position which corresponds to period (4).
In this instance, every time when the control moves out from an automatic focusing servo loop, the focus search is repeated. For this reason, once the control moves out from an automatic focusing servo loop, it takes time as experienced in the first time to return to the original focus condition. Moreover, if the position J (focusing position) of the just focus detection point can not be detected, the movement of the focus of the laser beams is repeated over a range between the lower most value and the upper most value which further requires time.
An object of the present invention is to resolve these conventional art problems and to provide an optical disk reproducing device which permits to restore in a short time from an out of focus servo to a focus servo condition.
An optical disk reproducing device according to the present invention which achieves the above object and which includes a laser beam source which irradiates laser beams onto an optical disk, a photo detector which receives the reflection beams from the optical disk and generates a position error detection signal with respect to a focus position error for the optical disk, a detection circuit which receives the position error detection signal and detects a focus position of the laser beam locating in a range of an automatic focus control loop, and a focus servo device which changes over to the automatic focus control loop in response to the detection signal of the detection circuit and performs an automatic focusing of the laser beams for the optical disk in response to the position error detecting signal, is characterized in that, the optical disk reproducing device further comprises a driver which controls the focus position of the laser beams in response to a driving signal, a search signal generation means which generates as the driving signal a search signal which continuously advances or backs the focus position of the laser beams with respect to the optical disk, a memory means which stores the focus position of the laser beams at the moment in response to the detection signal from the detection circuit or a signal output value of the search signal generation means corresponding to the focus position, a first control circuit which causes the search signal generation means to generate as a retry search signal a drive signal which moves the focus of the laser beams from positions ahead or back to the focus position toward the focus position based on the focus position of the laser beams stored in the memory means or the signal output value stored in the memory means, and a second control circuit which operates the first control circuit while releasing the automatic focus control loop when the detection signal from the detection circuit is stopped or when the detection signal with respect to the position error moves out from the focus position.
As has been explained above, the movement of the focus of the laser beams by the first control circuit is set at positions ahead or back to the focus position causing to enter into the automatic focus control loop, and the retry search signal for searching a focusing position is generated by the search signal gene/ration means, thereby, a refocusing control in case when an automatic focusing servo loop can not be maintained can be started from the positions ahead or back to the focus position causing to enter into the automatic focus control loop.
As a result, the control can immediately enter into the automatic focus control loop and can restore in a short time from an out of focus servo to a focus servo condition.